A liquid droplet ejection apparatus that performs printing by ejecting liquid droplets from a head is generally used for various industrial purposes as an inkjet printer. Applications of this industrial inkjet increases year by year, and the inkjet printer is used for not only performing printing on paper sheets, fabric, plastic sheets, and others but also performing printing of a design on a surface of a ceramic tile in recent years. Accordingly, performance that enables stably ejecting various kinds of inks for a long time has been demanded with respect to the liquid droplet ejection apparatus.
However, in case of performing printing by using as an ink a ceramic ink containing solid particles of ceramics or a white ink containing solid particles of a titanium oxide or the like as a pigment and ejecting liquid droplets from a head in which a plurality of ink chambers are aligned in an X direction or an XY direction, there is a problem that a nozzle of an ink chamber placed at an end portion in an alignment direction is clogged even if driving is effected to uniformly eject liquid droplets from the respective ink chambers. When ink clogging occurs in the nozzle of the ink chamber at the end portion, there is a phenomenon that ink clogging eventually likewise occurs in a nozzle of an inner ink chamber adjacent to this ink chamber and the nozzle clogging is propagated to the inner side. Since this phenomenon occurs even in case of a nonvolatile ink, it is a phenomenon different from nozzle clogging caused when a liquid is evaporated from a nozzle and dried.
As a result of keen examination conducted by the present inventors, a reason can be roughly considered as follows.
As shown in FIG. 1, in a head 1, an ink that is consumed by ejecting liquid droplets is supplied to respective ink chambers 11 aligned along the X direction in the drawing from a common ink chamber 13 communicating with the respective ink chambers 11. Although the ink in the common ink chamber 13 flows by the supply of the ink, flowability of the ink becomes poor around ink chambers 11b, 11b placed at end portions in the alignment direction as compared with the periphery of ink chambers 11a placed in a central portion in the alignment direction. That is because the ink around the ink chambers 11a in the central portion has high flowability when it flows toward the ink chambers 11a and ink chambers 11 on both sides thereof since the ink chambers 11 are arranged on both sides of the ink chambers 11a in the central portion, whereas the ink around the ink chambers 11b, 11b at both the end portions has lower flowability than that around the ink chambers 11a in the central portion since ink chambers are arranged only on the inner sides of the ink chambers 11b, 11b at both the end portions.
Since solid particles contained in a ceramic ink or a white ink have the higher specific gravity than regular color pigment particles, the solid particles in the ink are apt to settle out in a region having the low ink flowability as compared with a region having the high ink flowability. When the ink containing the solid particles that are apt to settle out is supplied to the ink chambers 11b, 11b at both the end portions, the solid particles settle out faster than in the ink chambers 11a in the central portion. As a result, when each nozzle 12 is arranged to be vertically downward directed as shown in FIG. 15, the solid particles S settle out near the nozzle 12 in the ink chamber 11, density of the solid particles 20 increases, and the nozzle clogging occurs.
Further, as such an ink, there is an ink that is used while being heated from an ordinary temperature to a predetermined temperature (e.g., 35° C. to 50° C.) by, e.g., arranging a heater (not shown) in the common ink chamber 13. When the ink is heated, its viscosity is lowered, and the ink can be easily flowed. In this case, since the ink chambers 11a in the central portion have the ink chambers 11 on both sides thereof, the vicinity of the ink chambers 11a is filled with the heated ink, an ink temperature is stable, but the ink near the ink chambers 11b, 11b at both the end portions has a low temperature and is apt to have high viscosity since the ink chambers are not provided on the outer side of these ink chambers. As a result, the flowability of the ink is lowered near the ink chambers 11b, 11b at both the end portions, and the solid particles in the ink are apt to settle out.
Furthermore, when the nozzles 12 of the ink chambers 11b, 11b at both the end portions are clogged, the ink is no longer supplied to these ink chambers 11b, 11b at both the ends, then the flowability of the ink around the ink chambers 11 adjacent to these ink chambers on the inner side is thereby lowered, and the ink clogging eventually occurs. It can be considered that the nozzle clogging is consequently gradually propagated toward the inner side.
Moreover, even when each nozzle 12 is arranged sideways, there is a problem that the solid particles in liquid droplets ejected from the nozzle 12 cannot have adequate concentration due to sedimentation of the solid particles and turbulence in ejection speed or non-uniformity of images is caused.
In the prior art, to reduce the sedimentation of solid substances such as a pigment in the ink, a technology that uses a pressure difference between a head and an ink tank to circulate an ink has been suggested (Patent Document 1). However, the ink on the head side that is circulated by this technology is an ink dedicated to a common chamber, and the ink that has been supplied to each ink chamber cannot be circulated. Therefore, the sedimentation of solid particles that occurs in the ink chambers cannot be suppressed at the time of printing pause.
As a countermeasure for the nozzle clogging at the time of printing pause, there is known a technology for applying a spare waveform to each ink chamber to vibrate a meniscus immediately before restarting ejection, thereby allowing an ink in the ink chambers to flow (Patent Document 2). However, this technology eliminates the nozzle clogging due to an increase in viscosity caused by evaporation of a volatile component in the ink. Since flow of the ink caused by such meniscus vibration is very small, this flow is effective for elimination of the nozzle clogging caused by the evaporation, but just slightly vibrating the meniscus cannot sufficiently eliminate a sedimentation state of the solid particles that has advanced in the ink chambers to some extent.
Moreover, there is also known that nozzle recovery is performed by performing a so-called flushing operation for forcedly discharging liquid droplets from the nozzle (Patent Document 3). However, this normalizes an increase in concentration of an ink due to evaporation of the ink by using preliminary ejection of the ink, and it does not avoid the nozzle clogging in an ink chamber at an end portion caused by sedimentation of solid particles having the specific gravity higher than that of a dispersion medium contained in the ink.